Rice is the most important food crop, and is the main source of food for more than half of the population all over the world (Khush, 2005). It is reported that an annual increase rate of 0.6 to 0.9% in rice yield is needed to meet the needs of people's lives (Carriger and Vallee, 2007). The increase of crop yield depends on farmland, and the sustainable development of agricultural production requires avoidance of environmental deterioration, destruction of ecological balance and loss of biodiversity (Cassman, 1999; Tilman et al., 2002). Crop yield is mainly determined by photosynthesis and respiration, however, both photosynthesis and respiration are sensitive to temperature (Yoshida, 1981), and also influenced by CO2 concentration in the atmosphere (Baker et al., 1990) and ozone layer (Maggs and Ashmore, 1998). These factors are also related to the greenhouse effect (Rosenzweig and Parry, 1994). Continual climate warming has brought devastating impacts on rice production. For two consecutive years 2006 and 2007, high temperature weather above 38° C. occurred for a long term in a large area along the middle and lower Yangtze River, including such as Chongqing, Hubei, Hunan, Anhui, Zhejiang, and Guangdong provinces, leading to large-scale drop of yield of rice and other crops in a large area, with ripening rate lower than 50% in the severe cases. Therefore, there is an urgent need for available heat tolerance genetic resources in breeding to respond to the serious threat of rice production caused by global warming. Therefore, it is of great significance in both theory and practice to carry out an extensive research on high temperature resistance, develop further heat tolerance genetic resource, and make efforts to select and breed new plant varieties with high temperature resistance, so as to respond to the serious threat of rice production caused by global warming.
In recent years, a large number of studies of genetic analysis and chromosomal mapping have been carried out at home and abroad regarding the high temperature resistance at the rice booting stage (Zhigang Zhao et al., 2006), heading stage (Yongsheng Zhang, et al., 2009; Zhang et al., 2009.), flowering stage (Liyong Zhao et al., 2003, 2002; Yi Pan et al., 2005; Qingquan Chen, 2008; Tao Zhang et al., 2008; Limei Que et al., 2008; Zhang et al., 2009; Jagadish et al., 2010.), filling stage (Changlan Zhu et al., 2005) and the stage for rice amylose synthesis and gel consistency formation (Changlan Zhu et al., 2006). In these studies, the mapped rice high temperature resistance related quantitative trait loci (referred to as QTL) relates to each chromosome of rice, wherein the QTL site with the most significant genetic effects is from variety Bala from Pakistan, which is responsible for 18% of phenotypic variation. However, since different researchers use different test materials and different high temperature treatment procedures and methods, the experimental data obtained are difficult to correspond to each other and reproduce, and the mapped QTL interval is relatively large, making it impossible to determine candidate genes and carry out the related molecular cloning and functional complementary verification. Therefore, the above mapped high temperature resistance related QTLs, not only theoretically lack support by the necessary data, but also are difficult to be used in practice effectively.
In addition, some researchers also carried out studies on rice high temperature resistance related genes by homologous cloning methods. Yamanouchi et al., (2002) used map-based cloning method for mapping, and cloned a rice spot gene Sp17. It is found that one of reading frame of the gene is highly similar with heat stress transcription factor. Under conditions of heat stress, the expression amounts of mutant and wild-type Sp17 are both up-regulated. Yokotani et al., (2008) transferred heat resistant gene encoding OsHsfA2e of rice into Arabidopsis thaliana and the tolerance of the transgenic A. thaliana to environmental stress is enhanced. According to the microarray analysis on transgenic A. thaliana plant with over-expression under non-stress condition exhibited increased expression amounts of some genes related to stress, including several types of heat shock proteins.
It is generally believed that heat shock proteins (HSPs) are associated with the high temperature response. It has been reported that rHsp90 responds to several stresses such as salt, drought and high temperature, and high temperature treatment at 42° C. and 50° C. for 30 minutes can significantly increase the rHsp90 expression amount (Liu et al., 2006). There is another report showing that there are 40 genes encoding proteins containing α-crystals, 23 of which are heat shock proteins (Sarkar et al., 2009). The microarray and RT-PCR analyses show that the expression amounts of 19 out of 23 heat shock proteins are up-regulated at high temperatures. In addition, Chang et al. (2007) transferred the rice heat shock protein Hsp101 into tobacco, and found that at high temperatures, the survival of the plant with over-expression is better than that of wild-type. Wu et al. (2009) drove OsWRKY11 expression with HSPIO promoter and found that the transgenic rice plants have relatively slower leaf wilting and a high survival rate after heat treatment. By over-expressing rice OsCEST (chloroplast protein capable of enhancing stress resistance) gene in A. thaliana, Yokotani et al. (2011) found that the transgenic plant is resistant to not only salt stress, but also is to drought and high temperature.
Zinc finger protein is a large family of transcription factors, which play important roles in gene expression regulation, cell differentiation, embryonic development and other biological processes (Gerisman & Pabo, 1997; Laity et al., 2001), especially in the expression regulation of stress related genes (Li & Chen, 2000). According to the number and location of cysteine (C) and histidine (H) residues in zinc finger protein, the transcription factors containing zinc-finger protein domains can be classified into subclasses C2H2, C2C2, C3H, C3HC4 (i.e., RING finger), C3HC5 (i.e., LM finger) and others. Among them, C2H2 zinc finger proteins are of the most clearly studied class among the zinc finger proteins, wherein two cysteines and two histidines form a coordinate bond with Zn2+, thereby forming in turn a tight finger structure containing one β fold and one α-helix. Kim et al. (2001) isolated a cold-inducible zinc finger protein gene SCOF-1 from the soybean cDNA library, which encodes a product containing two typical C2H2 zinc finger structures. The expression of SCOF-1 is specifically induced by cold and ABA, rather than salt stress. The transgenic study confirmed that the over-expression of SCOF-1 can enhance the cold resistance of A. thaliana and tobacco. Liu M. et al. (2007) cloned a soybean C2H2 zinc finger protein transcription factor gene GmC2H2, whose expression is related with the stress induction of cold and ABA. As for C3HC4 and CHY zinc finger proteins, the successful isolation are only reported in A. thaliana, rice, Physcomitrella patens, Artemisia desertorum, corn, pineapple, soybeans and other plants (Stone et al., 2005; Ohyanagi et al., 2006; Rensing et al., 2008; Yang et al., 2008; Alexandrov et al., 2009; Yang X. et al., 2009; Wu X. et al., 2010). A. desertorum AdZFP1 gene is a typical example that encodes such zinc finger proteins (Yang et al. 2008), and the semi-quantitative PCR analysis showed that, AdZFP1 gene is strongly induced by exogenous ABA, and to some extent is also induced by high salt, low temperature and high temperature. Wu X. et al. (2010) also screened a C3HC4 zinc finger protein gene GmRZFP1 from the cDNA library of the soybeans under drought condition, and the results demonstrated that the gene is mainly induced by high temperatures and drought stress. During the high temperature stress for 1-6 hours, the expression amount of GmRZFP1 Gene is positively correlated with treatment duration. In particular, under the high-temperature stress for 12 hours, the expression amount decreased, while the expression reached the highest level at 24 hours. These results therefore showed that GmRZFP1 gene is induced by a variety of stress treatments, probably involving in multiple stress signal transduction.
In addition, Huang et al. (2008) found 12 A20/AN1 type zinc finger proteins from the Japonica. The microarray analysis showed that the expressions of four genes (ZFP177, ZEP181, ZFP176, ZFP173), two genes (ZFP181 and ZFP176) and one gene (ZFP157) are induced by cold, drought and H2O2, respectively. Further study shows that ZFP177 responds to both low temperature and high temperature stress. By over-expressing ZFP177 gene, the obtained transgenic tobacco resists to low temperature of 2° C. and high temperature of 55° C., but becomes more sensitive to salt stress and drought stress, suggesting that ZFP177 plays an important role in various abiotic stresses in plant, while different stresses may have different response mechanisms.